(a) Field of the Invention
This invention relates to a wiper device for use in stripping fluid, such as drilling mud, from the outside surface of a well string.
(b) Prior Art
In my U.S. Pat. No. 3,733,641, issued May 22, 1973, which is incorporated herein by reference, there is disclosed a wiper device for stripping and recovering drilling mud or other fluid from the exterior surface of a well string. This wiper device incorporates a combination of components and functions which will now be described in a general way, in connection with a drilling operation.
The device comprises a box-like container. This container is normally mounted in the sub-structure directly beneath the rotary table of the drilling rig. The container has a central opening through which the vertical drilling string may pass. The opening is connected with the flow nipple extending up from the rig blow-out preventer. Drilling mud which is stripped from the string is retained by the container and flows downwardly through the opening into the flow nipple and is returned to the rig mud system.
In the container there is positioned a pair of horizontal wiper pads which close on the drilling string with pressure and strip off the mud. Each wiper pad is a flat member comprising a central wiper block, of rubber or like material, secured between upper and lower steel plates. The steel plates are recessed in the central leading edge section, so that it is only the wiper block which engages the drilling string. Each wiper block is formed with a horsehoe-shaped indentation in its leading edge, for providing encircling engagement with the drilling string. The wiper pads are disposed on opposite sides of the opening, in a vertically staggered arrangement.
Double-acting pneumatic cylinders are mounted on the wiper pads and link them together. The cylinders may be actuated to draw the pads together so that they close on the drilling string passing through the container opening. An air system supplies and exhausts operating air to and from the cylinders.
The side walls of the container are spaced outwardly from the side and end edges of the wiper pads. This, together with the mounting of the cylinders directly on the pads and the use of flexible air hoses, make it possible for the pads to move universally or in all directions in a horizontal plane. Thus they can move with the laterally swaying drilling string as it is being pulled from the well bore. Otherwise stated, the wiper pads are "free floating" with the drilling string.
An accumulator or pulsation tank is connected with the cylinder air supply and exhaust lines. The accumulator tank is included to permit the wiper pads to simultaneously move outwardly (when an enlargement, such as the drill bit, passes through them) while still maintaining a substantially constant closing pressure.
In summary, the assembly is designed to provide wiper pads which are free-floating, to accommodate lateral swaying of the drill string, and the air system is adapted to maintain a substantially constant wiping pressure.
(c) The Need
The prior art wiper device has been usable with the majority of rigs. However, there is a need for a wiper device which is capable of passing very large diameter components. A typical specification for a very large rig requires that a wiper device be capable of opening sufficiently to pass tools having a diameter up to 30 inches.
The prior art device of U.S. Pat. No. 3,733,641 is not amenable to being scaled up to this size. The width and length of the wiper components would be such that its rigidity would not be sufficient. Thicker plate and larger cylinders would be needed, which would affect the thickness of the unit, making it difficult to accommodate within the crowded confines of the sub-structure. The unitary assembly, consisting of the two wiper pads and the cylinders mounted on them, would be very heavy. This weight affects wear of the wiper block's horseshoe-shaped indentation surface. If the pad assembly is very heavy, the vertically travelling, swaying drilling string must bear heavily against the wiper block to cause the assembly to move laterally or longitudinally--this leads to rapid wear of the block. If the wear is at the end of the indentation, it is not a serious problem, as the cylinder will advance the block so that it tightens against the drilling string. But if the wear occurs at the side surfaces of the indentation, then there is no means by which the resultant gap can be remedied, and the wiping performance of the block suffers accordingly. This leads to having to replace the wiper block, which is not easy to do in the prior device, as it requires removal of the entire wiper pad and cylinder assembly for disassembly and insertion of a new block.
There is therefore a need for a new wiper device having minimal vertical thickness, wiper pads which can easily be moved laterally, and wiper pads whose resilient deformable wiper blocks may easily be replaced.